The present invention relates to a two cycle loop scavenged internal combustion engine having a piston reciprocal in a cylinder between a crankcase and a combustion chamber in a die cast cylinder block. The crankcase is typically carbureted or otherwise supplied with fuel, and fuel-air transfer passages extend between the crankcase and intake ports in the combustion chamber for supplying the fuel-air charge thereto.
In a loop scavenged engine, the direction of flow from the intake ports in the combustion chamber is critical. In contrast, the aiming of the intake charge in a cross flow type engine is not so critical because the piston in the cross flow engine has a deflection baffle in the combustion chamber.
In a loop scavenged engine, there typically are a pair of intake ports in the combustion chamber generally laterally opposed, with an exhaust port therebetween on one side, and a boost port therebetween on the other side. The transfer passages deliver the fuel-air flow to the intake ports, and the intake flow should optimally be directed from the intake ports towards each other to a common point generally distally opposite the exhaust port and proximate the boost port for circulation up and around the top of the combustion chamber to purge spent combustion products to the exhaust port.
The transfer passages extend within the cylinder block along the exterior wall of the cylinder and have respective die-tooling-receiving openings generally opposite the exterior cylinder wall to permit die casting of the block. Respective die cast port runner covers are inserted in the noted cylinder block openings. The port runner covers have a given configuration to shape the respective transfer passages.
The shapes of the transfer passages may be changed by changing the inner configuration of the port runner covers, and this is a typical design approach in attempting to improve performance. The main objectives in shaping the transfer passage are to provide the desired directivity and to provide smooth uniform undisturbed air flow. Thus, sharp corners or anything else that creates turbulence is avoided. Different area ratios are tried as the transfer passage gets smaller at its end toward the combustion chamber intake port so that the flow velocity is highest thereat. Various angles of intake flow are also experimented with to obtain optimum directivity.
It was during development efforts in shaping transfer passages, as above noted, that the present invention arose. During work with a Mercury 18 horsepower engine, various transfer passage shapes were tried, and the optimum shapes yielded corrected brake horsepower outputs of 18 horsepower. However, the corrected brake horsepower output would vary from engine to engine. For example, for the same shape transfer passage, the corrected brake horsepower output could vary by as much as 1 horsepower. The variation was seemingly unexplainable.
Numerous attempts with differently shaped transfer passages were made in order to try to both improve performance and narrow the horsepower variation window spread, without success.
Instead, it has been found that, while the shape of the transfer passage is important, there is another previously unrecognized factor causing the noted horsepower variation window. It is now recognized that the source of the variation problem is a short-circuiting path in the transfer passage which adversely affects directivity of flow into the combustion chamber from the intake port. This short-circuit path is along the exterior wall of the cylinder and is shorter than the flow path along the outermost portions of the transfer passage farthest away from the cylinder. The short-circuit flow directed a component of the intake flow toward the exhaust port and thus also detracted from performance.
Both horsepower and fuel economy have been improved, and the horsepower variation spread window has been narrowed, by enhancing combustion chamber intake directional flow control by directing flow along the outermost portions of the transfer passages, and blocking flow along shorter paths adjacent the cylinder which otherwise short-circuit the outermost flow and adversely affect directional flow into the combustion chamber from the intake ports, to substantially eliminate intake flow directed toward the exhaust port from the intake ports otherwise caused by the noted short-circuiting. In the noted Mercury 18 horsepower engine, both corrected brake horsepower and brake specific fuel economy have been increased by 10%, and the horsepower variation window has been substantially reduced to an almost negligible spread.